<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
</head>
<style>
body {
  width: 500px;
  height: 500px;
  background: url("tiger.jpg") no-repeat;
  background-size: 600px 500px;
}
main {
 position: relative;
      margin: 0 auto;
      padding: 1em;
      max-width: 23em;
      background: hsla(0,0%,100%,.25) border-box;
      overflow: hidden;
      border-radius: .3em;
      box-shadow: 0 0 0 1px hsla(0,0%,100%,.3) inset,
                  0 .5em 1em rgba(0, 0, 0, 0.6);
      text-shadow: 0 1px 1px hsla(0,0%,100%,.3);
}
main::before {
  /*background: url("http://csssecrets.io/images/tiger.jpg") 0 / cover fixed; 这个背景图是关键*/
  content: '';
  position: absolute;
  top: 0; right: 0; bottom: 0; left: 0;
  z-index: -1;
  filter: blur(20px);
}
/*借助blur() 滤镜，我们在CSS 中获得了对元素进行模糊处理的能力。*/
/*在使用负的z-index 来把一
个子元素移动到它的父元素下层
时，请务必小心：如果父元素的
上级元素有背景，则该子元素将
出现在它们之后。*/
</style>
<body>
  <main>
    <blockquote>
      "The only way to get rid of a temptation[...]"
      <footer>－
        <cite>
          Oscar Wilde,
          The Picture of Dorian Gray
        </cite>
      </footer>
    </blockquote>
  </main>
</body>
</html>